The invention relates to a pneumatic spring brake cylinder for vehicle brake systems. The spring brake cylinder has a housing, in which a spring brake piston which is loaded by way of at least one accumulator spring and is connected to a spring brake piston tube is guided movably, and has an emergency release device for emergency releasing of the spring brake piston if the pressure fails. The emergency release device contains a spindle which can be rotated with respect to the spring brake cylinder and can be screwed into a nut which is held fixedly in the spring brake piston tube so as to rotate with it by way of a positively locking connection and interacts with an axial stop on the spring brake piston, in order to cause a releasing movement of the spring brake piston counter to the action of the at least one accumulator spring.
A spring brake cylinder of this type usually has a housing, in which a spring brake piston is guided. The piston divides the interior of the housing into a spring accumulator chamber and a pressure space. A spring device which prestresses the spring brake piston in the direction of the pressure space is arranged in the spring accumulator chamber. In order that a parked or braked vehicle, in which the spring brake cylinder is arranged, can be moved, the pressure space is loaded with compressed air in such a way that the spring brake piston is displaced counter to the prestressing action of the accumulator spring. As a result, a brake device which is coupled to the spring brake piston is disengaged from a brake disk, with the result that the vehicle is no longer braked. For brake actuations during driving, a spring brake cylinder of this type can be coupled in a known way to a service brake cylinder in the form of what is known as a combined cylinder.
In the case of combined service brake and spring brake cylinders (combined cylinders), in the parking brake case the spring brake piston actuates a service brake piston of the service brake cylinder by way of its spring brake piston tube. The service brake piston in turn has a service brake piston rod which interacts with a brake mechanism of the vehicle brake, in particular a disc brake. A brake mechanism of this type is described, for example, in EP 0 740 085 B1 and has a pivoting lever which is connected in an articulated manner to the service brake piston rod and is connected fixedly to a brake application shaft so as to rotate with it. The brake application shaft is provided with a cam contour which, upon a rotation of the brake application shaft about its longitudinal axis, leads to an axial relative movement of a brake caliper and a brake pad of a disc brake.
If a substantial reduction in the air pressure in the pressure space then occurs on account of a pressure failure in the system, the spring brake piston is displaced in the direction of the pressure space on account of the prestress of the accumulator spring, in such a way that the brake device which is coupled to the spring brake piston acts and performs a braking operation of the vehicle. In this way, for example in the case of a line rupture of the brake pressure line during driving, it is to be reliably prevented that a commercial vehicle continues to drive in an unbraked manner and therefore becomes difficult to control.
This enforced braking operation in the case of a deficient compressed air feed is additionally also utilized in the case of trailers, etc., which have been parked for a relatively long time, in order that it can be prevented that said vehicles roll away unintentionally.
The braking action which is exerted in this way in cases of this type by way of the mechanical accumulator spring in the spring brake cylinder is to be capable of being released again, however, for example in order to displace parked vehicles or to clear a driving lane which is blocked on account of a breakdown, in order that the relevant vehicles can be moved and, for example, traffic obstacles can be eliminated.
Since the existing compressed air supply is sometimes damaged here, however, as a result of line rupture, etc. and cannot be used, and there is also no supplementary compressed air supply, it is necessary in practice that spring brake cylinders of this type have a mechanical emergency release device. The latter is usually provided with an actuating device in the form of a threaded spindle which is accessible from outside and can be actuated in a rotatable manner. The threaded spindle is held such that it can be rotated or screwed in a bush in the bottom of the spring brake cylinder and interacts, as described in WO 97/07322 A1 which forms the generic type, with a nut which is held fixedly in the spring brake piston tube so as to rotate with it. The result is that a rotation of the threaded spindle brings about a linear displacement of the nut on the threaded spindle.
Here, in this specific embodiment, the outwardly pointing end of the spring brake piston tube is provided with an inwardly protruding flange which acts as a stop for the nut. On account of the linear offset, which is introduced by way of the rotational movement of the threaded spindle, of the threaded spindle which is screwed on the nut in the axial direction, the nut comes into contact with the flange of the spring brake piston tube, a likewise linear displacement of the spring brake piston counter to the prestressing force of the accumulator spring occurring as a result of the further rotation of the threaded spindle. The result is that the brake device which interacts with the spring brake piston can be emergency released as a result.
As shown in FIG. 2 of WO 97/07322 A1, the nut is held fixedly in the spring brake piston tube so as to rotate with it by way of a positively locking connection, an external polygonal profile which runs around in the circumferential direction on the outer circumference of the nut, in particular a hexagonal profile, and a hexagonal profile of complementary configuration on the inner circumference of the spring brake piston tube, interacting in a positively locking manner.
This design has proven itself in practice with regard to its functionality. It has the disadvantage, however, that a sliding movement of the internal hexagonal profile of the spring brake piston tube with respect to the external hexagonal profile of the nut occurs in the case of each application of the spring brake which is induced by way of venting of the pressure space. Because the materials of the nut and the spring brake piston tube are not selected as a rule with regard to their sliding wear properties, material abrasion can occur in the case of a sliding movement of this type, it being possible for shavings to pass into the compressed air system and damage it.
In the case of a combined service and spring brake piston which is known from WO 2010/031512 A1, the nut of the mechanical emergency release device is held in a rotationally fixed manner by way of an intermediate bush which is arranged between the inner wall of the spring brake piston tube and the outer circumference of the nut. The intermediate bush is fixed in a rotationally fixed manner in a cutout of the spring brake piston rod by way of a radially outwardly pointing lug which is configured at its end which points toward the bottom of the spring brake cylinder. The lug, together with the cutout as anti-rotation safeguard, takes up a relatively large amount of installation space axially and radially, however.
In contrast, the present invention is based on the object of developing a spring brake cylinder of the type mentioned at the outset, in such a way that the above-described disadvantages are avoided.
This and other objects are achieved in accordance with embodiments of the invention.
The invention provides that the positively locking connection is formed firstly between an external polygonal profile which runs around in the circumferential direction on the outer circumference of the nut and a circumferential internal polygonal profile of complementary configuration on the inner circumference of an intermediate bush and, secondly, by way of an external polygonal profile which runs around in the circumferential direction on the outer circumference of the intermediate bush and a circumferential internal polygonal profile of complementary configuration on the inner circumference of the spring brake piston tube.
A polygonal profile is to be understood to mean an external or internal profile which, as viewed in cross section, has a plurality of corners or edges which are connected to one another by way of any desired surfaces, in particular straight, convex or concave surfaces.
As a result, firstly the material of the intermediate bush can be selected in such a way that the wear on account of a relative sliding movement of the spring brake piston tube with respect to the nut is reduced. The intermediate bush preferably consists of polyamide, in particular of glass fiber-reinforced polyamide, which has satisfactory sliding properties and high stiffness and strength on account of the relatively high torsional and shear loading which is introduced by way of the nut which is held in a rotationally fixed manner. As an alternative, the intermediate bush might also consist of a metal.
In contrast, the materials of the spring brake piston tube or the spring brake piston and/or the nut can be selected independently thereof with regard to other requirements, such as with regard to stiffness or weight.
Secondly, the positively locking connection between the nut and the spring brake piston tube by means of two polygonal profiles makes an extremely space-saving arrangement possible because polygonal profiles of this type, above all in the case of a relatively large number of edges, substantially enclose a cylindrical circumference and extend only slightly in the radial direction as a result. As a result, the wall thickness of the spring brake piston tube can be kept comparatively small.
The threaded spindle particularly preferably has an external thread which interacts with an internal thread of a through bore which is configured in a bottom of the housing, in order to screw the threaded spindle into and out of the housing. To this end, for example, a bush with an internal thread can be held in the bottom of the housing.
As an alternative, the threaded spindle might also, however, be held in a merely rotatable but axially fixed manner in the bottom of the housing. A solution of this type is described in WO 97/07322 A1 which was cited at the outset. Then, a rotation of the threaded spindle likewise causes screwing of the nut which is held in a rotationally fixed manner and therefore the axial movement of the nut.
The external polygonal profile which extends in the circumferential direction on the outer circumference of the intermediate bush and the circumferential internal polygonal profile of complementary configuration on the inner circumference of the spring brake piston tube are particularly preferably formed by way of a double-hex profile. As a result of the relatively high number of twelve edges, the radial extent of the profile is relatively small, which, as has already been mentioned above, has a positive effect on the installation space.
In particular, the double-hex profile can be configured in one piece with the spring brake piston tube. This is advantageous, above all, if the spring brake piston tube is configured in one piece with the spring brake piston and represents an injection molded product, for example made from plastic or aluminum. The double-hex profile can then be primary formed together with the spring brake piston/spring brake piston tube.
According to one development, the external polygonal profile which extends in the circumferential direction on the outer circumference of the nut and the circumferential internal polygonal profile of complementary configuration on the inner circumference of the intermediate bush can be formed by way of a hexagonal profile. Then, for example, a commercially available nut with an external hexagonal profile can be used.
The intermediate bush can be held in an axially fixed manner in the spring brake piston tube, for example between two axial stops, of which one stop is formed by way of a shoulder which extends radially to the inside from an inner wall of the spring brake piston tube and a further stop is formed by way of a disk which is fixed in the spring brake piston tube. In particular, the intermediate bush has at least an axial length which corresponds to the maximum relative movement between the nut and the spring brake piston tube, in order that the nut is held in a rotationally fixed manner in every position and/or in every screwing state. In other words, the intermediate bush has at least an axial length which corresponds to the axial screwing region of the nut on the threaded spindle.
The external polygonal profile which extends in the circumferential direction on the outer circumference of the intermediate bush and the circumferential internal polygonal profile of complementary configuration on the inner circumference of the spring brake piston tube particularly preferably extend substantially over the entire axial length of the intermediate bush. Then, the torsional moment and/or shear moment which act/acts on the intermediate bush from the nut can be supported over a relatively large length.
The axial stop for the nut is particularly preferably formed by way of a disk which is held in the spring brake piston tube. As an alternative, the axial stop for the nut might also be configured in one piece directly on the spring brake piston tube, for example by virtue of the fact that a shoulder which extends radially to the inside from the inner wall of the spring brake piston tube is configured on the spring brake piston tube, which shoulder then forms the axial stop for the nut.
The invention also relates to a combined service brake and spring brake cylinder for brake systems of vehicles, having the above-described spring brake cylinder.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.